1. Field of the Invention
The present invention relates to a semiconductor device controlling refresh operations of a plurality of memory areas, and relates to a system thereof.
2. Description of Related Art
Generally, in a semiconductor device such as DRAM having a data storage function, it is required to refresh memory cells in a predetermined time interval in order to retain data thereof. There are two refresh modes instructed to the semiconductor device by a controller, which include, for example, auto-refresh and self-refresh. The controller issues a plurality of auto-refresh commands during a predetermined time in the auto-refresh. The controller issues one self-refresh command in the self-refresh. The self-refresh command may be an entry command to enter the self-refresh or an exit command to exit the self-refresh. The semiconductor device performs a refresh operation in the predetermined time interval during a period from the entry command to the exit command. Specifically, the self-refresh is performed using one internal timer (oscillator) provided in the semiconductor device during the above period. In the semiconductor device, a request signal (refresh start signal) for performing the refresh is generated at every predetermined time by using the internal timer. The number of memory cells to be refreshed in response to one request signal is generally larger than the number of memory cells to be refreshed in response to one auto-refresh command. This is due to the fact that semiconductor manufacturers who are familiar with characteristics of the memory cells particularly adjust time setting values of the internal timer in accordance with the characteristics of the memory cells.
Meanwhile, semiconductor devices having a plurality of memory areas are widely used in recent years with an increase in capacity of the semiconductor devices such as DRAM. The plurality of memory areas can have various forms. For example, a semiconductor device comprising a memory cell array divided into a plurality of memory banks (hereinafter, referred to as “a first semiconductor device”) and a stacked type semiconductor device in which a plurality of memory chips are stacked (hereinafter, referred to as “a second semiconductor device”) can be achieved. In these semiconductor devices, when the respective memory areas are refreshed simultaneously in a self-refresh operation, there arises a problem that large noise is generated by temporal concentration of operation currents in the self-refresh operation. As measures against such a problem, a control method has been proposed in which refresh operations are performed for a plurality of memory banks at different timings (for example, see Patent References 1 and 2), and a control method has been proposed in which refresh operations are performed for a plurality of stacked memory chips at different timings (for example, see Patent References 3 and 4). Here, the plurality of memory banks corresponds to the plurality of memory areas capable of operating asynchronously with one another. The plurality of memory banks included in the first semiconductor device transmit data from/to outside through a common I/O data port (input/output data port) at timings different from one another. Further, the plurality of memory banks receive command and address information corresponding to the plurality of memory areas from outside through a common command port and a common address port at timings different from one another. The command information includes, for example, active/precharge commands for activating/precharging each memory bank, and read/write commands for reading/writing each memory bank. The plurality of memory chips in the second semiconductor device operate through the common I/O data port, the common command port and the common address port, similarly as in the first semiconductor device. However, when managing a system including the semiconductor device, the self-refresh is characterized that the system does not access the semiconductor device entirely within a predetermined time, which is a function that the semiconductor device retains stored data by itself within the predetermined time. Thus, the self-refresh command is a command that deals with the plurality of memory areas (the plurality of memory banks in the first semiconductor device and the plurality of chips in the second semiconductor device) in an integrated manner. Specifically, the controller (included in the system) controlling the semiconductor device issues a first command as a self-refresh entry for starting the self-refresh to the semiconductor device. After the predetermined time, the controller issues a second command as a self-refresh exit for terminating the self-refresh to the semiconductor device. The semiconductor device receives the first and second commands through the above common command port and controls the plurality of memory areas at different timings respectively. Thus, in terms of the self-refresh, the plurality of memory areas are controlled at different timings in the first and second semiconductor devices, which is a sequential delay control synchronized with one self-refresh command.    [Patent Reference 1] Japanese Patent Application Laid-open No. H7-122065    [Patent Reference 2] Japanese Patent Application Laid-open No. H2001-35152    [Patent Reference 3] Japanese Patent Application Laid-open No. 2006-277870 (U.S. Pat. No. 7,466,577)    [Patent Reference 4] Japanese Patent Application Laid-open No. 2007-140948 (U.S. Pat. No. 7,894,293)
The inventors have reviewed a third semiconductor device having a plurality of memory areas capable of being self-refreshed independently of and asynchronously with one another. The third semiconductor device is capable of performing, for example, a self-refresh operation in a first memory area, a read operation in a second memory area, a write operation in a third memory area, and an idle (standby) state in a fourth memory area. Further, the third semiconductor device is capable of shifting the second memory area performing the read operation to a state of the self-refresh being the same as the first memory area. In other words, the third semiconductor device is common to the first and second semiconductor devices in that the plurality of memory areas perform active, precharge, read and write operations asynchronously with one another, but is obviously different from the first and second semiconductor devices in that the plurality of memory areas can perform self-refresh operations asynchronously with one another. Specifically, in the third semiconductor, the plurality of memory areas are controlled independently of one another in response to a plurality of self-refresh commands received asynchronously with one another. As described above, the conventional method of the self-refresh control is the sequential delay control synchronized with one self-refresh command. In other words, the plurality of memory areas are commonly controlled in response to common self-refresh commands (the first and second commands). However, even when applying the methods of the self-refresh control disclosed in the Patent References 1 to 4 to the third semiconductor device reviewed by the inventors, noise suppression cannot be achieved. Specifically, when the plurality of memory areas shift to the state of the self-refresh independently of and asynchronously with one another, there arises a problem that the noise due to overlapping of operations cannot be effectively suppressed. Thus, when reviewing the third semiconductor device, it is required to reduce noise associated with the self-refresh, the amount of which is larger than that of the auto-refresh.